No Moon? (Potentially) No Problem for the Habitability of Alien Planets.

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It has long been assumed that the the effect of our moon’s gravitational pull is largely responsible for the habitability of our planet. Model work had suggested that without the moon’s ability to stabilize Earth, the planet’s axial tilt, or obliquity, would vary greatly, causing climatic swings too harsh for life to combat.

Despite this long held perspective, Jack Lissauer of the Ames Research Center has a different idea. Lissauer says that “if the Earth did not have a moon, its obliquity — and, therefore, its climate — [it] would vary, indeed, substantially more than it does at present… but it’s nowhere near as bad as was predicted based on previous models.” So take that, traditional thought.

One important effect of the moon is that it keeps the Earth’s axial tilt, or its “obliquity”, at somewhere between 22 and 24.6 degrees from its vertical position. Our four seasons are the product of Earth’s axial tilt and their relatively mild climatic variations are the product of this small range of obliquity. While the height of summer in New York City feels like a completely different brand of terrible than the height of winter in New York City, the fact that life can survive in both seasons speaks to the relative stability of Earth’s climate throughout a given year.

The moon plays a huge role in the stable climate of Earth but it may not mean the difference between life and, well, no life. (NASA)

Some scientists feel that the stabilizing effect of the moon is so important that without it, Earth would be lifeless. This assertion is based on modeling work that has put the variation of the moon-less Earth’s axial tilt between 0 and 85 degrees. A 0-degree tilt would have the sun constantly overhead at the equator and would provide the poles with close to no light. An 85-degree tilt would flip our planet over, giving the two poles far more extreme variations in daylight than they have now. In this scenario, the Northern Hemisphere would receive alternating 6 months periods of daylight and darkness, causing similarly extreme temperature shifts. Kind of like how,in San Francisco, it’s really cold in the shade, and then as soon as you put on your sweater you realize it’s actually a little too hot in the sun. That effect times a billion.

The arctic midnight sun. Traditional models have suggested that without a moon, conditions like the round the clock brightness of a polar summer would be even more intensified. (Jeremy Potter NOAA/OAR/OER)

But Lissauer’s team decided to design a model with a different consideration factored in – long stretches of time. They created a simulation of a moon-less earth and ran the model over a simulated 4-billion years. Over this more comprehensive time frame, the team found that the Earth’s tilt did vary far more than it does with a moon, but far less than previous models had suggested. For example, in a 100-million year period, the axial tilt never got up to 40 degrees or down to 10 degrees.

“We’re not talking about, really, the Earth without the moon as a realistic model for the Earth, unless somebody goes out there and destroys the moon,” Lissaur said. (Looking at you, Vader.) “We’re using this as the first case of studying a plausible exoplanet, and we’re going to use some future calculations — we’re going to do the same thing with other systems.”

Though Lissaur seems convinced that a moon-less planet could support life, he and his team remain unsure of what space wolves would howl at should they exist on such a planet.

Are you sold on the idea of a habitable moon-less planet? If life could survive there, would the temperature shifts still be too great for it to ever achieve an advanced state? Do you guys know what I mean by that annoying sun-shade temperature shift in San Fran? The worst, right? Let us know below.

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Mars has an axial tilt (23 degrees) similar to that of Earth’s and it doesn’t have a large moon to stabilize it. Phobos and Deimos, the two martian moons, are nothing more than glorified asteroids, to small to have any discernible effect on the axial tilt.